Signal acquisition probe storing compressed or compressed and filtered time domain impulse or step response data for use in a signal measurement system

ABSTRACT

A signal acquisition probe stores compressed or compressed and filtered time domain data samples representing at least one of an impulse response or step response characterizing the signal acquisition probe. The compressed or compressed and filtered time domain data samples of the impulse response or the step response are provided to a signal measurement instrument for compensating the signal measurement instrument for the impulse or step response of the signal measurement instrument.

The present invention related generally to signal acquisition probes andmore particularly to a signal acquisition probe storing compressed orcompressed and filtered time domain data samples of an impulse responseor a step response characterizing the voltage through response of thesignal acquisition probe.

Signal acquisition probes are designed to measure electrical signal fromDC to greater than 20 GHz. They are also designed to meet specificrequirements of a user, such as measuring voltage or current signals.Voltage probes are divided into passive voltage probes, active voltageprobes, differential voltage probes, high voltage probes and lowcapacitance probes. Each type of probe has a specified bandwidth that isused with a signal measurement instrument, such as an oscilloscope,logic analyzer and the like, having a sufficiently wide bandwidth toallow accurate measurements of electrical signals from a device undertest.

The frequency bandwidths of a signal acquisition probe and a signalmeasurement instrument are specified as above the 3 dB down point of thefrequency magnitude. Generally, the combination of the probe andoscilloscope in a measurement system results in the system bandwidthbeing lower than the bandwidths of the individual probe andoscilloscope. Various techniques have been used to peak the overallbandwidth of the oscilloscope and probe system.

U.S. Pat. No. 6,725,170 describes a smart probe that is used with anoscilloscope for automatic self-adjustment of the oscilloscope'sbandwidth. The probe contains a memory in which is stored S-parametersthat characterize the probe's frequency response. When the probe isconnected to the oscilloscope, the S-parameters of the probe are readinto the oscilloscope. The oscilloscope has a controller that isoperable under program control for automatically adjusting the scope'sfrequency response to compensate for the probe's characteristicfrequency response, whereby the scope and probe when connected arecharacterized by a predefined overall system frequency response.

The amount of memory required to store the S-parameters characteristicsof the measurement probe increases as a function of the bandwidthsprobe. This results in increased costs and size of the probe. What isneeded is a way of storing the response characteristics of measurementprobe without substantially increasing the cost or size of the probe.

SUMMARY OF THE INVENTION

Accordingly, the present invention is to a signal acquisition probehaving a memory therein for storing compressed or compressed andfiltered time domain data samples representative of an impulse responseor a step response characterizing the signal acquisition probe. Thesignal acquisition probe includes a housing having a probing tipextending from one end of the housing and an electrically conductivecable extending from the other end of the housing. Electrical circuitryis disposed within the housing and coupled to the probing tip and theelectrically conductive cable. The memory stores time domain datasamples of a through impulse response or a through step response.

The memory may also store sets of compressed or compressed and filteredtime domain data samples of impulse responses of the signal acquisitionprobe wherein the sets includes at least two of a forward throughimpulse response, reverse through impulse response, input reflectionvoltage impulse response and output reflection voltage impulse response.Likewise, the memory may store sets of compressed or compressed andfiltered time domain data samples of step responses of the signalacquisition probe wherein the sets includes at least two of a forwardthrough step response, reverse through step response, input reflectionvoltage step response and output reflection voltage step response. Thecompressed or compressed and filtered time domain data samples of atleast one of the impulse responses and step responses may be derivedfrom at least one of S-parameters, T-parameters, and ABCD-parametersrepresenting a frequency response of the signal acquisition probe.

The electrical response of a signal acquisition probe may becharacterized by applying a calibration signal to the signal acquisitionprobe and acquiring time domains data samples of the calibration signalcharacterizing the response of the signal acquisition probe. Acompressed time domain data sample record is generated from the timedomain data samples of the characterizing response of the probe. Thecompressed time domain data sample record of the characterizing responseis stored in a memory associated with the signal acquisition probe. Thecalibrating signal may be a rising edge step signal or an impulsesignal. The acquired time domain data samples of the rising edge stepsignal represent a characterizing step response of the probe. Thederivative of the time domain data samples representing thecharacterizing step response of the probe may be taken to generate timedomain data samples representing an impulse response of the probe.

The acquired time domain data samples characterizing the response of thesignal acquisition probe may be compressed as a function of a valuerepresenting the rate of change of magnitude values of the time domaindata samples exceeding a threshold value. The acquired time domain datasamples characterizing the response of the signal acquisition probe mayalso be compressed as a function of comparing magnitude values of thetime domain data samples to at least one of a maximum threshold valueand minimum threshold value. The acquired time domain data samplescharacterizing the response of the probe may also be filtered.

The filtering of the acquired time domain data samples representing thecharacterizing response of the signal acquisition probe includesgenerating an averaged time domain data sample value over a time domaindata sample range from an initial time domain data sample to asubsequent time domain data sample from the time domain data samplerecord when the rate of change value exceeds the threshold value.Additional averaged time domain data sample values are generated overadditional time domain data sample ranges where the subsequent timedomain data sample from the preceding time domain data sample rangebecomes the initial time domain data sample for the next time domaindata sample range. The averaged time domain data samples from the timedomain data sample ranges are stored as a filtered and compressed timedomain data sample record representing the characterizing response ofthe signal acquisition probe in the memory associated with the signalacquisition probe.

A further method of filtering of the time domain data sample recordrepresenting the characterizing response of the signal acquisition probeincludes generating an averaged time domain data sample value over atime domain data sample range defined by an initial time domain datasample and a subsequent time domain data sample exceeding at least oneof a maximum threshold value and a minimum threshold value centered onthe initial time domain data sample. Additional averaged time domaindata sample values are generated over additional time domain data sampleranges where the subsequent time domain data sample from the precedingtime domain data sample range becomes the initial time domain datasample for the next time domain data sample range. The averaged timedomain data samples from the time domain data sample ranges are storedas filtered and compressed time domain data sample record representingthe characterizing response of the signal acquisition probe in thememory associated with the signal acquisition probe.

One method of generating time domain data samples representative of animpulse response or a step response of the signal acquisition probe isto acquire at least one of S-parameter reflection coefficients andtransmission coefficients characterizing the spectral response of thesignal acquisition probe. The S-parameter reflection or transmissioncoefficient is converted to time domain data samples representing animpulse response. The time domain sample data are compressed as afunction of the rate of change of the impulse response and stored inmemory of the signal acquisition probe.

Additional steps may include acquiring the S-parameter reflectioncoefficients and transmission coefficients characterizing the spectralresponse of the signal acquisition probe and converting each of theS-parameter reflection coefficients and the transmission coefficients totime domain data samples representing corresponding impulse responses.The time domain data samples of the each of the impulse responses arecompressed as a function of the rate of change of the respective impulseresponses. The compressed time domain data samples of the respectiveimpulse responses are stored in memory of the signal acquisition probe.

Each of the impulse responses representing one of the S-parametertransmission coefficient and reflection coefficients may be converted totime domain data samples of corresponding step responses. The timedomain data samples of the each of the step responses are compressed asa function of the rate of change of the respective step responses. Thecompressed time domain data samples of the respective step responses arestored in memory of the signal acquisition probe.

The objects, advantages and novel features of the present invention areapparent from the following detailed description when read inconjunction with appended claims and attached drawings.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a perspective view of a signal measurement system having asignal acquisition probe storing compressed time domain impulse or stepresponse data samples according to the present invention.

FIG. 2 is a block diagram of a signal measurement system having a signalacquisition probe storing compressed time domain impulse or stepresponse data samples according to the present invention.

FIG. 3 is a graphical representation of a step response characterizing asignal acquisition probe for illustrating the use of the rate of changein voltage samples for compressing time domain voltage samples in a timedomain step response waveform record.

FIG. 4 is a graphical representation of an impulse responsecharacterizing a signal acquisition probe for illustrating the use ofthe rate of change in voltage samples for compressing time domainvoltage samples in a time domain impulse response waveform record.

FIG. 5 is a graphical representation of a step response characterizing asignal acquisition probe for illustrating the use of a maximum andminimum threshold value range around voltage samples for compressingtime domain voltage samples in a time domain step response waveformrecord.

FIG. 6 is a graphical representation of an impulse responsecharacterizing a signal acquisition probe for illustrating the use of amaximum and minimum threshold value range around voltage samples forcompressing time domain voltage samples in a time domain impulseresponse waveform record.

FIG. 7 is a graphical representation of a step response characterizing asignal acquisition probe for illustrating the use of a maximum andminimum threshold value range around voltage samples for filtering andcompressing time domain voltage samples in a time domain step responsewaveform record.

FIG. 8 is a graphical representation of an impulse responsecharacterizing a signal acquisition probe for illustrating the use of amaximum and minimum threshold value range around voltage samples forfiltering and compressing time domain voltage samples in a time domainimpulse response waveform record.

FIGS. 9A-9B is a flow chart describing the steps in convertingS-parameter data into impulse response waveform records and stepresponse wave form records in signal measurement system having a signalacquisition probe storing compressed or compressed and filtered timedomain impulse or step response data samples according to the presentinvention.

FIG. 10 is a flow chart describing the steps in compensating a signalchannel of a signal measurement instrument connected to a signalacquisition probe storing compressed or compressed and filtered timedomain impulse or step response data samples according to the present.

FIGS. 11A-11B are a flow chart describing alternate steps incompensating a signal channel of a signal measurement instrumentconnected to a signal acquisition probe storing compressed or compressedand filtered time domain impulse or step response data samples accordingto the present.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a signal acquisition probe 10 connected to a signalmeasurement instrument 12, such as a digital oscilloscope, logicanalyzer or the like, forming a signal measurement system 14 for testinga circuit or device under test. The signal acquisition probe 10 includesa housing 16, in the form of a probe head, with probing tips 18extending from one end of the housing 16 and a signal cable 20 extendingfrom the other end. The probing tips 18 may be signal and ground probingtips or differential probing tips. The housing may also include a singleprobing tip for coupling a signal under test to the probe. The housing16 includes electrical circuitry for conditioning an electrical signalfrom a device under test. The signal cable 20 is coupled to a probeinterconnect housing 22. The probe interconnect housing 22 hasadditional electronic circuitry, such as amplifiers, attenuators,filters, memory and the like, that may be controlled by a controllerwithin the probe 10. The controller receives commands from theoscilloscope 12 that are interpreted by the controller for settingvariable parameters in the probe, such as gain, attenuation, offsetvoltage and the like. The signal cable 20 preferably includes a singlecoaxial wire having a central signal conductor and a surrounding groundor shield conductor. The cable 20 further includes power lines thatprovide the appropriate voltages to the electronic circuitry in theprobe head. The cable 20 may also include one or more command lines thatprovide command signals from the controller to the electrical circuitryin the probe head. The probe interconnect housing 22 is removablyconnected to one of several interconnect receptacles 24 on the frontpanel 26 of the signal measurement instrument 12. The interconnectreceptacles 24 have electrical interconnects for coupling the signalunder test to signal channels in the signal measurement instrument 12,coupling voltages from the signal measurement instrument 12 to thesignal acquisition probe 10, and coupling commands and data to and fromthe signal acquisition probe 10 and signal measurement instrument 12.

Referring to FIG. 2, there is shown a representative block diagram ofthe signal measurement system 14 having signal acquisition probes 10containing compressed time domain impulse or step response data samplesaccording to the present invention. Like elements from FIG. 1 arelabeled the same in FIG. 2. The signal measurement system 14 includesthe signal measurement instrument 12 which for the purposes ofdescribing the invention is a digital oscilloscope. The digitaloscilloscope 12 has multiple interconnect receptacles 24 disposedtherein for receiving interconnect housings 22 of one or more signalacquisition probes 10 as representatively shown in FIG. 2. Each of theinterconnect receptacles 24 is coupled to a signal channel 28 in thesignal measurement instrument 12. The signal acquisition probe 10 has aprobe head in the form of the housing 16 with at least one probing tip18 extending from one end. The housing includes electrical circuitry 30,such as attenuation circuitry, amplification circuitry and the like,that is coupled to receive a signal under test via the probing tips 18.The output of the electrical circuitry 30 is coupled via the signalcable 20 to the probe interconnect housing 22.

The probe interconnect housing 22 includes a probe controller 32 whichmay include an embedded memory 34 or a separate memory device coupled tothe controller 32. The probe controller 32 has a communications bus 36,such as an I²C bus, IEEE 1494 bus, USB bus or the like, that providesbi-directional communications with the signal measurement instrument 12.Preferably the communications bus includes a clock line and a data linecoupled to a probe interconnect housing interface 38. The output of thesignal cable 20 is coupled to probe circuitry 44, such as amplifiers,attenuators, filters, offset circuitry, and the like. The probecircuitry 44 operates on the signal under test to condition the signalunder test for processing by the digital oscilloscope 12. Generally, theprobe circuitry 44 is controlled by the probe controller 32. The outputof the probe circuitry 44 is coupled to the digital oscilloscope 12 viaa signal interconnect 46, such as a blind mate connector, BNC connectoror the like, in the probe interconnect housing interface 38. The probeinterconnect housing interface 38 also includes a voltage powerinterconnect 40 for providing voltage power to the signal acquisitionprobe 10 from a power supply 42 in the digital oscilloscope 12.

The digital oscilloscope 12 has a controller 48 that is coupled to acommunications bus 50 that is coupled to the probe interconnect housinginterface 38. The communications bus 50 provides bi-directionalcommunications with the signal acquisition probe 10 and may take theform of an I²C bus, IEEE 1494 bus, USB bus or the like. Preferably thecommunications bus 50 includes a clock line and a data line coupled tothe probe interconnect housing interface 38.

The digital oscilloscope controller 48 is coupled via a system bus 52 tomemory 54. The memory 54 represents both RAM, ROM and cache memory withthe RAM memory storing volatile data, such as digital data samples of aninput signal generated by an acquisition system 56 coupled to receivethe acquired signal from the signal acquisition probe 10. Eachacquisition system 56 is associated with one of the signal channels 28of the signal measurement instrument. The system bus 52 is alsoconnected to the display device 58, such as a liquid crystal display,cathode ray tube or the like, and to front panel controls 60 which mayinclude control entry devices, such as a keyboard and/or mouse as wellas the knobs and buttons. A mass storage unit or units 62, such as ahard disk drive, CD ROM drive, tape drive, floppy drive or the like thatreads from and/or writes to appropriate mass storage media, may also beconnected to the system bus 52. Program instructions for controlling thedigital oscilloscope 12 may be stored and accessed from the ROM memory54 or from the mass storage media of the mass storage unit 62. Thecontroller 48 in the above described digital oscilloscope 12 may also beimplemented using multiple controllers and digital signal processingdevices. For example, a second controller, such as a Power PCmicroprocessor manufactured and sold by Motorola, Inc., Schaumburg,Ill., may be included to control the acquisition and processing of theacquired signal. The display device 58 may be controlled by a displaycontroller receiving display instructions from a digital oscilloscopecontroller 48 and receiving display data from a digital signalprocessing device. A bus controller may also be included to monitor theprobe interconnect housing interface 38 for connected signal acquisitionprobes 10, and provide communications between the probe interconnecthousing interface 38 and the controller 48.

Referring to FIG. 3, there is shown a graphical representation of a stepresponse 70 characterizing a signal acquisition probe 10. A fast risingstep voltage is applied to the probing tip 18 of the signal acquisitionprobe 10 and the output of the probe 10 is acquired by a samplinginstrument, such as a sampling oscilloscope. The sampling oscilloscopecaptures voltage samples at discrete time locations, as shown by thedots 72 on the step response 70, covering the fast rising step voltage.High resolution sampling is used to acquire the voltage samples so thataberrations 74 in the step response 70, such as roll off, overshoot,ringing and the like, can be characterized. The time domain voltagesamples over time are stored as time domain data samples in a stepresponse waveform record.

FIG. 4 shows a graphical representation of an impulse response 76characterizing a signal acquisition probe 10. An electrical impulse maybe generated using an optical source generating a narrow opticalimpulse, on the order of femtoseconds, that is converted to anelectrical signal by a photodiode. The electrical impulse is applied tothe probing tip 18 of the signal acquisition probe 10 and the output ofthe probe 10 is acquired by a sampling instrument, such as a samplingoscilloscope. The sampling oscilloscope captures voltage samples atdiscrete time locations, as shown by the X's 78 on the impulse response76. High resolution sampling is used to acquire the voltage samples sothat aberrations in the impulse response 76 can be characterized. Thevoltage samples over time are stored as time domain data samples in animpulse response waveform record. Alternately, the impulse responsewaveform record of the signal acquisition probe 10 may be generated bytaking the derivative of the time domain data samples representing thecharacterizing step response of the signal acquisition probe.

The time domain data samples of the step response in FIG. 3 and theimpulse response in FIG. 4 can be compressed or compacted into acompressed time domain data sample record to reduce the amount ofcharacterizing time domain data samples stored in probe memory 34. Oneway to compress the time domain data samples is to calculate a slopebetween the initial data sample voltage of one or the other of the stepresponse or impulse response waveform records and its adjacent datasample voltage to determine a rate of change between the two data samplepoints. If the slope between the two data sample voltages is less than athreshold slope value, then the initial data sample voltage is saved aspart of the compressed time domain data sample record and the adjacentdata sample voltage is not. The saved data sample voltages are shown bythe circles around the dots in the step response of FIG. 3 and thecircles around the X's in the impulse response of FIG. 4. The slopebetween the initial data sample voltage and the next data sample voltageof the step response or impulse response waveform record is calculatedand compared against the threshold slope value. If the slope between thetwo data sample voltages is less then the threshold slope value, thenthis next data sample voltage is not saved. The process of calculatingthe slope between the initial data sample voltage and the data samplevoltages in the step response or impulse response waveform records andcomparing the slopes to the threshold slope value continues until one ofthe calculated slopes exceeds the threshold slope value. The data samplevoltage used with the initial data sample voltage to generate the slopeexceeding the threshold slope value is saved as part of the compressedtime domain data sample record. This saved data sample voltage thenbecomes the initial data sample voltage and is used with the next datasample voltage for calculating the slope between the two voltagesamples.

If the calculated slope between the new initial data sample voltage andthe next adjacent data sample voltage is greater than the thresholdslope value, then the adjacent data sample voltage is saved as part ofthe compressed time domain data sample record. The adjacent data samplevoltage becomes the new initial data sample voltage and the slopebetween the new initial data sample voltage and the next data samplevoltage is calculated. If the slope between the two data sample voltagesis greater than the threshold slope value, then the adjacent data samplevoltage is saved as part of the compressed time domain data samplerecord. The process of calculating the slope between initial data samplevoltages and other data sample voltages in the step response or impulseresponse waveform records, comparing the slopes to the threshold slopevalue and saving the data sample voltages from the slopes exceeding thethreshold slope value as part of the compressed time domain data samplerecord continues until the ends of the step response or impulse responsewaveform records. The resulting compressed time domain data samplerecord of the step or impulse response waveform record is stored in thememory 34 of the signal acquisition probe 10.

FIGS. 5 and 6 are respective graphical representations of a stepresponse and an impulse response characterizing a signal acquisitionprobe 10 illustrating an alternative embodiment for compressing orcompacting the time domain data samples to reduce the amount ofcharacterizing time domain data samples stored in probe memory 34. Amaximum and minimum threshold value range is defined and positioned overthe data sample voltages of the respective step and impulse responsewaveform records. Preferably, the maximum and minimum threshold range iscentered on the data sample voltage but may be offset above or below themidpoint of the range. The adjacent data sample voltage in the stepresponse or impulse response waveform record is compared to the maximumand minimum threshold range centered on the initial data sample voltage.If the adjacent data sample voltage is within the maximum and minimumthreshold range, then the initial data sample voltage is saved as partof the compressed time domain data and the adjacent data sample voltageis not as shown by the max-min lines above and below the dots and X's inFIGS. 5 and 6. The next data sample voltage in the step response orimpulse response waveform records is compared to the maximum and minimumthreshold range centered on the initial data sample voltage. If the nextdata sample voltage is within the maximum and minimum threshold rangecentered on the initial data sample voltage, then the next data samplevoltage is not saved. The process of comparing the maximum and minimumthreshold range centered on the initial data sample voltage to the datasample voltages in the step response or impulse response waveformrecords continues until one of the data sample voltages exceeds maximumand minimum threshold range centered on the initial data sample voltage.The data sample voltage exceeding the maximum and minimum thresholdrange centered on the initial data sample voltage is saved as part ofthe compressed time domain data sample record. The maximum and minimumthreshold range is centered on the saved data sample voltage whichbecomes the initial data sample voltage and is used to compare with thenext data sample voltage for determining if the next data sample voltageexceeds the maximum and minimum threshold range centered on the new datasample voltage.

If the next adjacent data sample voltage exceeds the maximum and minimumthreshold range centered on the new initial data sample voltage, thenthe adjacent data sample voltage is saved as part of the compressed timedomain data sample record. The adjacent data sample voltage becomes thenew initial data sample voltage with the maximum and minimum thresholdrange centered on this new initial data sample voltage. The nextadjacent data sample voltage is compared to the maximum and minimumthreshold range centered on this new initial data sample voltage. If thenext adjacent data sample voltage is greater than the maximum andminimum threshold range centered on this new initial data samplevoltage, then the adjacent data sample voltage is saved as part of thecompressed time domain data sample record. The process of comparingmaximum and minimum threshold range centered on new initial data samplevoltages to subsequent data sample voltages in the step response orimpulse response waveform records and saving data sample voltagesexceeding the new initial data sample voltages continues until the endsof the step response or impulse response waveform records. The resultingcompressed time domain data sample record of the step or impulse isstored in the memory 34 of the signal acquisition probe 10.

The time domain data sample records of the step response or the impulseresponse may be filtered to reduce noise associated with the acquisitionof the time domain data samples. Referring to FIGS. 7 and 8 illustratingrespective graphical representations of a step response and an impulseresponse characterizing a signal acquisition probe 10, an adaptiveboxcar filter is applied to the respective step response and impulseresponse using the maximum and minimum threshold range centered on datasample voltages. Data sample voltages in the respective step responsewaveform record or the impulse response waveform record are compared tothe maximum and minimum threshold range centered on the initial datasample voltage in the waveform record. When a subsequent data samplevoltage exceeds the maximum and minimum threshold range of the initialdata sample voltage, the adaptive boxcar filter averages the magnitudesof the data sample voltages by the number of data samples between theinitial data sample voltage and the subsequent data sample voltageexceeding the maximum and minimum threshold range of the initial datasample voltage as shown by region labeled 80. The time interval of theadaptive boxcar filter between the initial data sample voltage and thesubsequent data sample voltage exceeding the maximum and minimumthreshold range of the initial data sample voltage is divided in halfand the averaged data sample voltage magnitude value is preferablyassigned this time value. Interpolation between data samples may be usedto accurately position the averaged data sample voltage at the propertime location in the region 80. The averaged data sample voltagemagnitude at the calculated time is stored as part of the compressed andfiltered time domain data sample record. The subsequent data samplevoltage exceeding the maximum and minimum threshold range becomes thenew initial data sample voltage and subsequent data sample voltages ofthe step response or impulse response waveform record are compared tothe maximum and minimum threshold range centered on the new initial datasample voltage. Subsequent data sample voltages in the waveform recordare compared to the maximum and minimum threshold range centered on thenew initial data sample voltage to determine the next data samplevoltage that exceeds the maximum and minimum threshold range. Theadaptive boxcar filter averages the magnitudes of the data samplevoltages by the number of data samples between the new initial datasample voltage and the subsequent data sample voltage exceeding themaximum and minimum threshold range of the new initial data samplevoltage as shown by region labeled 82. The time interval of the adaptiveboxcar filter between the new initial data sample voltage and thesubsequent data sample voltage exceeding the maximum and minimumthreshold range of the new initial data sample voltage is divided inhalf and the averaged data sample voltage magnitude is preferablyassigned this time value. The new averaged data sample voltage magnitudeat the new calculated time is stored as part of the compressed andfiltered time domain data sample record. The adaptive boxcar filtercontinues through the step response or impulse response waveform recordsdetermining filter regions (e.g. 84, 86, 88, . . . 122), averaging thedata sample voltages within the regions, determining time values, andstoring the averaged data sample voltage magnitudes as part of thecompressed and filtered time domain data sample record,

The above adaptive boxcar filtering process has been described withrespect to a maximum and minimum threshold range around data samplevoltages. The same adaptive boxcar filter may equally be implementedusing a rate of change threshold value as previously described forcompressing the step response and impulse response waveform records. Athreshold slope value is set and the slope is calculated between theinitial data sample voltage of the step response waveform record orimpulse response waveform record and subsequent data sample voltages inthe record to determine the rate of change between the two data samples.When the slope between the two data sample voltages exceeds thethreshold slope value, the adaptive boxcar filter averages themagnitudes of the data sample voltages by the number of data samplesbetween the initial data sample voltage and the subsequent data samplevoltage. The time interval of the adaptive boxcar filter between theinitial data sample voltage and the subsequent data sample voltage isdivided in half and the averaged data sample voltage magnitude ispreferably assigned this time value. Interpolation between data samplesmay be used to accurately position the averaged data sample voltage atthe proper time location. The averaged data sample voltage magnitude atthe calculated time is stored as part of the compressed and filteredtime domain data sample record. The adaptive boxcar filter continuesthrough the step response or impulse response waveform recordsdetermining the regions where the slope between data sample voltagesthat exceed the threshold slope value, averaging the data samplevoltages within the regions, determining time values, and storing theaveraged data sample magnitude voltages as part of the compressed andfiltered time domain data sample record.

The present invention has been described where the step response andimpulse response data samples are acquired in the time domain using asampling oscilloscope. Alternately, the impulse response and the stepresponse may be generated from a frequency response of the signalacquisition probe 10. The signal acquisition probe 10 is coupled tofrequency verification system, such as a vector network analyzer, andthe frequency response of the signal acquisition probe 10 ischaracterized. The vector network analyzer provides a series of signalsat various frequencies to the signal acquisition probe 10 and theS-Parameter responses of the signal acquisition probe 10 at the variousfrequencies are captured. The S-Parameters represent scattering ratiosof a multi-port network, such as a four-port network, with S₁₁ and S₂₂representing reflection coefficients and S₂₁ and S₁₂ representingtransmission coefficients. The S-parameters of the signal acquisitionprobe 10 characterizes the frequency response of the probe 10 in thespectral domain and may be stored as a Touchstone file. The S-parametersmay be converted to T-parameters, ABCD parameters and the like. AnInverse Fast Fourier Transform or the like may be applied to each of theS₂₁ and S₁₂ transmission coefficients and S₁₁ and S₂₂ reflectioncoefficients of the S-Parameters to generate sets of impulse responsesrepresenting the S₂₁ and S₁₂ transmission coefficients and the S₁₁ andS₂₂ reflection coefficients. The S₂₁ transmission coefficients areconverted to forward through impulse response waveform record. The S₁₂transmission coefficients are converted to reverse through impulseresponse waveform record. The S₁₁ reflection coefficients are convertedto input reflection voltage impulse response waveform record and the S₂₂reflection coefficients are converted to output reflection voltageimpulse response waveform record. One or all of the various impulseresponse waveform records may be stored in the probe memory 34.Integrating the various impulse response waveform records generatecorresponding step response waveform record with one or all of the stepresponse waveform records being stored in the probe memory 34.Interpolation may be used on the voltage samples of both the impulseresponse waveform records and the step response waveform records toproduce voltage samples at any desired time steps.

The flow chart of FIGS. 9A-9B shows the steps in converting S-parameterdata into time domain impulse response waveform records and time domainstep response waveform records. The signal acquisition probe 10 isconnected to the vector network analyzer and at least one of theS-parameter reflection and transmission coefficients are acquired forcharacterizing the spectral response of the signal acquisition probe asshown at step 140. Generally, the vector network analyzer acquired theS-parameters for all of the transmission coefficients and all of thereflection coefficients. At least one of the S-parameter transmissioncoefficients or reflection coefficients are converted to time domaindata samples representing the impulse response using an Inverse FastFourier Transform or the like at step 142. The time domain data samplesof the impulse response may be compressed or compressed and filtered asrepresented by decision step 144. The time domain data samples of theimpulse response are filtered and compressed as a function of the rateof change of the impulse response or as a function of the maximum andminimum threshold value range applied to the time domain data samples atstep 146. The time domain data samples of the impulse response arecompressed as a function of the rate of change of the impulse responseor as a function of the maximum and minimum threshold value rangeapplied to the time domain data samples at step 148. The compressed timedomain data samples of the impulse response or the compressed andfiltered time domain data samples of the impulse response are stored inthe probe memory 34 at step 150.

The time domain data samples of one or all of the impulse responses maybe converted to time domain data samples representing the step responsesby integrating time domain data samples representing the impulseresponses at step 152. The time domain data samples of the step responseor responses may be compressed or compressed and filtered as representedby decision step 154. The time domain data samples of the step responseor responses are compressed and filtered as a function of the rate ofchange of the step response or as a function of the maximum and minimumthreshold value range applied to the time domain data samples at step156. The time domain data samples of the step response or responses arecompressed as a function of the rate of change of the step responses oras a function of the maximum and minimum threshold value range appliedto the time domain data samples at step 158. The compressed orcompressed and filtered time domain data samples of the step response orresponses are stored in the probe memory 34 at step 160.

The compressed time domain data samples or the compressed and filteredtime domain data samples of the step response or the impulse responsestored in memory 34 of the signal acquisition probe 10 may be used forcompensating a signal channel of the signal measurement instrument 12.FIG. 10 is a flow chart describing the steps in compensating a signalchannel 28 in the signal measurement instrument 12 using the compressedtime domain data samples or the compressed and filtered time domain datasamples of the step response or the impulse response characterizing thesignal channel 28 of the signal measurement instrument 12. Thecompressed time domain data samples or the compressed and filtered timedomain data samples of at least one of the step response or the impulseresponse is stored in memory 34 of the signal acquisition probe 10 asshown in step 170. The signal acquisition probe 10 is connected to oneof the interconnect receptacles 24 of the signal measurement instrument12. The controller 48 in the signal measurement instrument 12 senses theconnection of the signal acquisition probe 10 and sends a command to theprobe controller 32 requesting the contents of the probe memory 34 to beprovided to the signal measurement instrument 12. The compressed timedomain data samples or compressed and filtered time domain data samplesof at least one of the step response or impulse response stored inmemory 34 is transferred to the signal measurement instrument 12 at step172. The compressed time domain data samples or the compressed andfiltered time domain data samples are interpolated at step 174 toprovide sufficient data samples for use by the controller 48 incomputing a compensation filter. The controller 48 computes acompensation filter from the compressed time domain data samples or thecompressed and filtered time domain data samples of the step response orthe impulse response and the time domain data samples characterizing thesignal channel 28 of the signal measurement instrument 12 at step 176.The compensation filter may take the form of an Infinite Finite Response(FIR) filter having multiple filter taps that compensates for at leastone of the impulse response or step response of the signal acquisitionprobe 10.

Once the signal channel 28 of the signal measurement instrument 12 hasbeen calibrated, the signal acquisition probe 10 may be connected to adevice under test to acquire a signal under test. The signal acquisitionsystem 56 of the signal channel 28 acquires time domain data samples ofthe signal under test at step 178. The time domain data samplesgenerated by the acquisition system 56 are processed using the computedcompensation filter at step 180. The processed time domain data samplesusing the compensation filter may be displayed on the display device 58of the signal measurement instrument 182.

The flow chart of FIGS. 11A-11B shows an alternate method ofcompensating the signal channel 28 of the signal measurement instrument12. The time domain data samples characterizing the signal channel 28 ofthe signal measurement instrument 12 are stored in the signalmeasurement instrument 12 as spectral domain data at step 190. Thecompressed time domain data samples or the compressed and filtered timedomain data samples of the step or impulse responses of the signalacquisition probe 10 are loaded into the signal measurement instrument12 and may be interpolated by the controller 46 in the signalmeasurement instrument 12 to generate equal time step between samples atstep 192 for providing a sufficient number of time domain data samplesfor conversion to the spectral domain. The interpolated time domain datasamples of the impulse response are converted to a spectral domainrepresentation using a Fast Fourier Transform or the like at step 194.The derivative of the time domain data samples of the step responseproduces an impulse response which may be converted to a spectralresponse a Fast Fourier Transform or the like. The controller 48computes a spectral domain compensation filter from the spectral domaindata samples of the step response or the impulse response and thespectral domain data samples characterizing the signal channel 28 of thesignal measurement instrument at step 196. Once the spectral domaincompensation filter is computed for the signal channel 28 of the signalmeasurement instrument 12, the signal acquisition probe 10 may beconnected to a device under test to acquire a signal under test. Thesignal acquisition system 56 of the signal measurement instrument 12acquires time domain data samples of the signal under test at step 198.The time domain data samples generated by the acquisition system 56 areconverted to the spectral domain at step 200 and processed using thecomputed spectral domain compensation filter at step 202. The spectraldomain data samples of the signal under test processed by the spectraldomain compensation filter are converted to compensated time domain datasamples at step 204. The processed compensated time domain data samplesmay be displayed on the display device 58 of the signal measurementinstrument at step 206.

The present invention has been described using the rate of change andmaximum and minimum range for compressing or compressing and filteringthe time domain data samples of a step response or an impulse response.The present invention is not limited to these two examples and othercompression and compression and filtering processed may be used. Forexample, the adaptive boxcar filter may be replaced with a Gaussianweighted average filter or a raised cosine filter. Additionally, alow-pass filter may be applied to the time domain data samples todetermine sample points, and then an adaptive filtering algorithm may beapplied to filter the original signal based on the sample pointsdetermined from the low-pass filter waveform. Further, the samplespacing and window width for filtering could be determined by simplymaking the width a function of the distance from the midpoint of therising edge of the step response or the peak of the impulse response.The filter keeps every sample within a certain time of the center of thestep response or impulse response waveform record (assuming the edge iscentered in the record), then filters everything outside that time.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments of thisinvention without departing from the underlying principles thereof. Thescope of the present invention should, therefore, be determined only bythe following claims.

What is claimed is:
 1. A signal acquisition probe comprising: a housinghaving a probing tip extending from one end of the housing, the probingtip configured to acquire time domain data samples to characterize afrequency response of the signal acquisition probe, and an electricallyconductive cable extending from the other end of the housing withelectrical circuitry disposed within the housing and coupled to theprobing tip and the electrically conductive cable; a controllerconfigured to compress the time domain data samples into a compressedtime domain data sample record, as a function of a value representingthe rate of change of magnitude values of the time domain data samples,when the rate of change value exceeds a threshold value; a memoryassociated with the signal acquisition probe configured to store thecompressed time domain data sample record derived from acquired timedomain data samples representative of a step response of the signalacquisition probe, wherein the compressed time domain data samplerecords include at least a first step response and a second stepresponse selected from the group including a forward through stepresponse, reverse through step response, input reflection voltage stepresponse and output reflection voltage step response of the signalacquisition probe, wherein the first step response and the second stepresponse are different and the first step response and the second stepresponse characterize the frequency response of the signal acquisitionprobe.
 2. The signal acquisition probe as recited in claim 1 wherein atleast one of the compressed time domain data sample records andcompressed and filtered time domain data sample records of the stepresponse are derived from at least one of S-parameters, T-parameters,and ABCD-parameters representing the frequency response of the signalacquisition probe.
 3. The signal acquisition probe as recited in claim1, further comprising a filter configured to filter the time domain datasamples prior to compression.
 4. A signal acquisition probe comprising:a housing having a probing tip extending from one end of the housing,the probing tip configured to acquire time domain data samples tocharacterize a frequency response of the signal acquisition probe, andan electrically conductive cable extending from the other end of thehousing with electrical circuitry disposed within the housing andcoupled to the probing tip and the electrically conductive cable; acontroller configured to compress the time domain data samples into acompressed time domain data sample record, as a function of a valuerepresenting the rate of change of magnitude values of the time domaindata samples, when the rate of change value exceeds a threshold value; amemory associated with the signal acquisition probe configured to storethe compressed time domain data sample record derived from acquired timedomain data samples representative of an impulse response of the signalacquisition probe, wherein the compressed time domain data samplerecords include at least a first impulse response and a second impulseresponse selected from the group including a forward through impulseresponse, reverse through impulse response, input reflection voltageimpulse response and output reflection voltage impulse response of thesignal acquisition probe, wherein the first impulse response and thesecond impulse response are different and the first impulse response andthe second impulse response characterize the frequency response of thesignal acquisition probe.
 5. The signal acquisition probe as recited inclaim 4 wherein at least one of the compressed time domain data samplerecords and compressed and filtered time domain data sample records ofthe impulse response are derived from at least one of S-parameters,T-parameters, and ABCD-parameters representing the frequency response ofthe signal acquisition probe.
 6. The signal acquisition probe as recitedin claim 4, further comprising a filter configured to filter the timedomain data samples prior to compression.